Readiness Monitors
Structured self-tests performed by a vehicle's OBD-II system to verify that all emissions control systems are functioning properly.
Readiness monitors are built-in self-diagnostic routines performed by a vehicle's On-Board Diagnostics II (OBD-II) system. Their primary purpose is to verify that the vehicle's emissions control systems are operating correctly and that the computer has run all its self-tests. The OBD-II system uses up to eleven diagnostic monitors, which are categorized as either continuous or non-continuous.
Continuous monitors—such as the misfire monitor, fuel system monitor, and comprehensive component monitor—run constantly while the engine is running. Non-continuous monitors—such as the catalytic converter, oxygen sensor, evaporative emissions (EVAP), and EGR system monitors—only run under specific driving conditions. Once a monitor completes its self-test without errors, its status is set to "Ready" or "Complete."
Readiness monitors are critical for state vehicle emissions inspections. If you clear a check engine light or disconnect the vehicle's battery shortly before an inspection, all readiness monitors are reset to "Not Ready." Most inspection stations will reject a vehicle if too many monitors are incomplete, even if the check engine light is off. The vehicle must be driven to complete the self-tests.
To make the monitors run and set to "Ready," the vehicle must be driven through a specific drive cycle—a combination of cold starts, idling, stop-and-go driving, and steady highway speeds. Technicians use OBD-II scan tools to check monitor status before sending a vehicle for testing.